A passive containment cooling system of a conventional nuclear power plant will be outlined with reference to FIGS. 5 to 8.
FIG. 5 is a sectional elevation view showing an example of the configuration of a conventional passive containment cooling system. As shown in FIG. 5, a core 1 is housed inside a reactor pressure vessel 2. The reactor pressure vessel 2 is housed in a containment vessel 3.
The inside of the containment vessel 3 is divided into a dry well 4 which houses the reactor pressure vessel 2, and a wet well 5. The dry well 4 and the wet well 5 constitute part of the containment vessel 3. In the wet well 5, a suppression pool 6 is formed. Above the suppression pool 6, a wet well gas phase 7 is formed.
In the case of a boiling water reactor, an atmosphere of the containment vessel 3 is inerted with nitrogen, and oxygen concentration is so limited as to be low. Moreover, in the case of a boiling water reactor, the containment vessel 3 is housed in a nuclear reactor building 100.
Furthermore, although not shown in FIG. 5, outside the containment vessel 3, a fuel pool 35 and a dryer and separator pool 38 are provided (see FIG. 4). The fuel pool 35 is a pool that stores spent fuel. The dryer and separator pool 38 is a pool that temporarily stores core internals (dryers and steam-water separators) at a time of refueling.
In general, in terms of material, there are various types of containment vessel 3, including a steel containment vessel, a reinforced concrete containment vessel (RCCV), a pre-stressed concrete containment vessel (PCCV), and a steel concrete composite (SC composite) containment vessel (SCCV). In the case of RCCV and PCCV, an inner surface is normally lined with a steel liner. FIG. 5 shows an example of RCCV.
From the reactor pressure vessel 2, a main steam pipe 71 extends outside the dry well 4. On the main steam pipe 71, a safety relief valve (SRV) 72 is provided. A discharge pipe 73 is so provided as to be submerged in the suppression pool 6, thereby enabling the steam in the reactor pressure vessel 2 to be released into the suppression pool 6 when the safety relief valve 72 is activated.
The dry well 4 and the suppression pool 6 are connected together via LOCA vent pipes 8. A plurality of LOCA vent pipes 8, e.g. ten LOCA vent pipes 8, are placed. However, FIG. 5 shows only two LOCA vent pipes 8. The LOCA vent pipes 8 constitute part of the containment vessel 3.
In order to allow gas in the wet well gas phase 7 to flow back to the dry well 4, vacuum breakers 9 are provided. A plurality of vacuum breakers 9, e.g. eight vacuum breakers 9, are placed. However, FIG. 5 shows only one vacuum breaker 9.
The following methods are available to place the vacuum breakers 9: a method of placing the vacuum breakers 9 on a wall surface of the wet well 5; a method of placing the vacuum breakers 9 on a ceiling of the wet well 5; and a method of placing the vacuum breakers 9 on the LOCA vent pipes 8. The vacuum breakers 9 are activated and opened when the pressure inside the wet well 5 is higher than the pressure inside the dry well 4, and the differential pressure exceeds a set pressure. For example, the set pressure of the vacuum breakers 9 is about 2 psi (about 13.79 kPa). The vacuum breakers 9 constitute part of the containment vessel 3.
On top of the dry well 4, a containment vessel head 10, which can be removed at a time of refueling and is made of steel, is provided. The containment vessel head 10 constitutes part of the containment vessel 3. In recent years, there is a type in which water is stored on the containment vessel head 10 so that a water shield 11 is provided and used as a shield during a normal operation.
Outside the containment vessel 3, a cooling water pool 13 of a passive containment cooling system 12 is provided. Inside the cooling water pool 13, cooling water 14 is stored. FIG. 5 shows an example of tank type of the cooling water pool 13. However, a pool type is also available. In the case of a pool type, an upper portion thereof is covered with a lid. In the example shown in FIG. 5, the cooling water pool 13 and the like are placed inside the reactor building 100. However, the cooling water pool 13 and the like may be placed inside an adjacent auxiliary building and the like.
An exhaust port 15 is provided to release steam to the environment from a gas phase above a water surface of the cooling water pool 13. At the outlet of the exhaust port 15, an insect screen may be provided. The cooling water pool 13 is generally placed above the containment vessel 3. However, the cooling water pool 13 may be provided beside the containment vessel 3.
Inside the cooling water pool 13, a heat exchanger 16 is so placed that at least part of the heat exchanger 16 is submerged in the cooling water 14.
In many cases, several heat exchangers 16 are provided. However, FIG. 5 shows only one heat exchanger. The heat exchanger 16 includes an inlet plenum 17, an outlet plenum 18, and a plurality of heat exchanger tubes 19 (see FIG. 6).
In the example shown in FIG. 5, only the heat exchanger tubes 19 are placed inside the cooling water pool 13, and the inlet plenum 17 and the outlet plenum 18 are projecting out of the cooling water pool 13. However, the configuration is not limited to that shown in the example. For example, there is also an example in which the entire heat exchanger 16, including the inlet plenum 17 and the outlet plenum 18, is placed inside the cooling water pool 13.
To the inlet plenum 17, a gas supply pipe 20 is connected to supply the gas from the dry well 4. One end of the gas supply pipe 20 is connected to the dry well 4.
To the outlet plenum 18, a condensate return pipe 21 and a gas vent pipe 22 are connected. One end of the condensate return pipe 21 is connected to the inside of the containment vessel 3. In FIG. 5, as one example, one end of the condensate return pipe 21 is led into the LOCA vent pipe 8. However, the configuration is not limited to that of the example. In other examples, one end of the condensate return pipe 21 may be led into the dry well 4, or to the suppression pool 6.
One end of the gas vent pipe 22 is led into the wet well 5, and is so placed as to be submerged in the suppression pool 6. The gas vent pipe 22 is less submerged in the suppression pool 6 than the top end of the opening of the LOCA vent pipe 8 in the suppression pool 6.
FIG. 6 is a sectional elevation view illustrating an example of a heat exchanger of a conventional passive containment cooling system. With reference to FIG. 6, the configuration of the heat exchanger 16 of the conventional passive containment cooling system 12 will be described with the use of an example of a horizontal heat exchanger.
In FIG. 6, the outlet plenum 18 is provided under the inlet plenum 17. A plurality of U-shaped heat exchanger tubes 19 are connected to a tube plate 23. The straight piping sections of the heat exchanger tubes 19 are placed horizontally. What is shown in FIG. 6 is simplified, and only two tubes are shown. A space outside the heat exchanger tubes 19 is filled with the cooling water 14 (see FIG. 5). The inlets of the heat exchanger tubes 19 are opened to the inlet plenum 17. The outlets of the heat exchanger tubes 19 are opened to the outlet plenum 18.
To the inlet plenum 17, the gas supply pipe 20 is connected. The gas supply pipe 20 supplies a mixed gas in the dry well 4, such as nitrogen, hydrogen, and steam, to the inlet plenum 17. The mixed gas is led into the heat exchanger tubes 19, and steam is condensed and turned into condensate. The condensate flows into the outlet plenum 18 from the outlets of the heat exchanger tubes 19, and is accumulated in a lower portion of the outlet plenum 18.
To a lower portion of the outlet plenum 18, the condensate return pipe 21 is connected, allowing the condensate in the outlet plenum 18 to flow back into the containment vessel 3 by gravity. To an upper portion of the outlet plenum 18, the gas vent pipe 22 is connected. A non-condensable gas, such as nitrogen and hydrogen, which is not condensed in the heat exchanger tubes 19, is discharged from the heat exchanger tubes 19, and is accumulated in an upper portion of the outlet plenum 18.
The tip of the gas vent pipe 22 is led to the suppression pool 6. The non-condensable gas in the outlet plenum 18 passes through the gas vent pipe 22, pushing down the pool water in the suppression pool 6 and vented into the pool water. Then, the non-condensable gas moves to the wet well gas phase 7.
By the way, the shape of the heat exchanger tubes 19 is not limited to a U-shape. There is also a structure in which a heat exchanger tube 19 having a vertical straight piping section is placed vertically as disclosed in Japanese Patent Application Laid-Open Publication No. 09-184897, the entire content of which is incorporated herein by reference. The inlet plenum 17 must be placed above the outlet plenum 18 to lead the condensate that is condensed in the heat exchanger tubes 19 to the outlet plenum 18 by gravity. The advantage of the horizontal type is seismic resistance and effective use of the cooling water 14. On the other hand, the advantage of the vertical type is good drainage of the condensate.
The following describes a function of the conventional passive containment cooling system 12 having the above configuration.
If a loss-of-coolant accident (LOCA), in which a pipe is broken in the dry well 4, occurs, steam is generated from the reactor pressure vessel 2, causing rapid pressure rise in the dry well 4. Then, the gas (mainly including nitrogen and steam) in the dry well 4 passes through the gas supply pipe 20 of the passive containment cooling system 12, and is supplied to the heat exchanger 16.
The non-condensable gas accumulated in the outlet plenum 18 of the heat exchanger 16 passes through the gas vent pipe 22, and is vented to the suppression pool 6. The venting of the non-condensable gas is driven by pressure difference between the dry well 4 and the wet well 5.
At the time of the LOCA, the pressure inside the dry well 4 is higher than the pressure in the wet well 5. Therefore, the non-condensable gas is vented smoothly. As a result, after a while, most of the gas inside the dry well 4 becomes only steam. In this state, the heat exchanger 16 is able to efficiently condense the steam inside the dry well 4 and return the condensate to the containment vessel 3.
However, immediately after the LOCA occurs, a large amount of steam is generated from coolant and the gas inside the dry well 4 is rapidly vented into the wet well 5 mainly through the LOCA vent pipe 8.
The steam is condensed in the suppression pool 6. The non-condensable nitrogen is not condensed in the suppression pool 6, and moves to the wet well gas phase 7. Due to the rapid venting from the LOCA vent pipe 8, after the LOCA, it only takes about one minute to transfer most of the nitrogen inside the dry well 4 to the wet well 5, for example.
After that, the vent flow becomes smaller. Since the gas vent pipe 22 is so set as to be less submerged in the suppression pool 6 than the LOCA vent pipe 8, the gas inside the dry well 4 is vented into the wet well 5 via the gas vent pipe 22 after a certain period of time has passed since the LOCA.
In this manner, as the vent flow calms down, and the steam generated by decay heat of a core fuel and released into the dry well 4 from a break of the LOCA is designed to be led to the heat exchanger 16 through the gas supply pipe 20, and to be cooled, without passing through the LOCA vent pipe 8.
As a result, the decay heat of the core fuel is transmitted to the external cooling water 14. Thus, it is possible to prevent pressurization of the containment vessel 3 caused by heat up of the water in the suppression pool 6. In that manner, the passive containment cooling system 12 is so designed as to be able to passively cool the containment vessel 3 without using any external power.
With reference to FIG. 7, the safety reinforcement measures for existing reactors under study will be described. They are used or the case where transients, such as a station blackout (also referred to as “SBO,” hereinafter), should occur. FIG. 7 is a sectional elevation view schematically illustrating an example of safety reinforcement measures for a conventional nuclear power plant.
When a transient, such as a station blackout (SBO), occurs, it is impossible to supply power from an offsite power 41 and an emergency diesel generator 42. Therefore, an entire active emergency core cooling system, which requires power from the emergency diesel generator 42, cannot be operated.
For some existing reactors, the following safety reinforcement measures are studied: even in the event that a station blackout occurs, a pump 46 of a high pressure core spray system (HPCS) is used to inject water into a core 1 by using the suppression pool 6 and the like as water sources. For example, the measures include diversification of power sources by an additional gas turbine generator 44 placed on a hill 43; and diversification of cooling of the pump 46 by an air cooling system 45 such as an air fin cooler.
In the following description, the diverse high pressure core spray system is referred to as diverse HPCS (DHPCS) 47. The diverse HPCS 47 has a function of injecting water into the core 1. However, the diverse HPCS 47 does not have a function of releasing the decay heat generated from the core fuel out of the containment vessel 3.
The following describes a conventional filtered venting system with reference to FIG. 8. A filtered venting system 50 has been employed in nuclear power plants in Europe after the accident at the Chernobyl nuclear plant.
FIG. 8 is a sectional elevation view illustrating a design example of a conventional filtered venting system. The filtered venting system 50 includes: a filtered venting vessel 51 which stores decontamination water 52; an inlet pipe 53 which leads the gas inside the containment vessel 3 into the decontamination water 52; and an outlet pipe 54 which releases the gas of a gas phase of the filtered venting vessel 51 to the environment.
The filtered venting vessel 51 and the like are placed not only in a building but also at other locations. If the filtered venting vessel 51 and the like are additionally placed for existing reactors, the filtered venting vessel 51 and the like are often placed outside a reactor building. If the filtered venting vessel 51 and the like are installed at the start of construction, the filtered venting vessel 51 and the like may be placed inside a reactor building or the like.
There is a type of the filtered venting vessel 51 in which a Venturi scrubber 55 is placed inside the decontamination water 52, and the gas led from the inlet pipe 53 passes through the Venturi scrubber 55. However, the Venturi scrubber 55 is not necessarily required. There is also a type in which a metal fiber filter 56 is placed in a gas phase of the filtered venting vessel 51. However, the metal fiber filter 56 is not necessarily required.
FIG. 8 shows the case where both the Venturi scrubber 55 and the metal fiber filter 56 are provided. As one example, on the inlet pipe 53, isolation valves 57a and 57b are provided in series; a rupture disk 58 is provided in parallel to the above components. Before and after the rupture disk 58, isolation valves 59a and 59b, which are normally open, are provided.
An outlet valve 60 is provided on the outlet pipe 54. In the conventional filtered venting system, in order to take in the gas in the containment vessel 3, one end of the inlet pipe 53 is connected directly to the containment vessel 3.
FIG. 8 shows one inlet pipe 53. However, in general, the following example is common: the inlet pipe 53 is divided into two, one of which is connected to penetration 28 (see FIG. 1) of the dry well 4 of an atmospheric control system outlet pipe 27 for an atmospheric control system, and the other one of which is connected to penetration 34 (see FIG. 3) of the wet well 5 of an atmospheric control system outlet pipe 27a for an atmospheric control system. The isolation valves 57a and 57b, the rupture disk 58, and the isolation valves 59a and 59b are provided for each of the divided pipes.
In this case, the atmospheric control system is generally installed to allow the atmosphere inside the containment vessel 3 to be inerted with nitrogen or to return to the air atmosphere. In general, the atmospheric control system includes an inlet pipe, which is used to inject gas into the containment vessel 3, and an outlet pipe, which is used to discharge the gas out of the containment vessel 3.
In the atmospheric control system, an atmospheric control system outlet pipe 27 is connected to the penetration 28 of the containment vessel 3. On the atmospheric control system outlet pipe 27, a first isolation valve 29 is provided. The other end of the atmospheric control system outlet pipe 27 is connected to the inlet pipe 53 of the filtered venting vessel 51.
An atmospheric control system outlet pipe 27a that branch off from the atmospheric control system outlet pipe 27 is connected to the penetration 34 of the wet well 5. On the atmospheric control system outlet pipe 27a, a first isolation valve 29a is provided (See FIG. 3).
A containment spray system is generally provided for pumping up the suppression pool water and delivering spray water in the dry well 4 at the time of an accident in order to cool the inside of the containment vessel 3 and to decontaminate radioactive materials. In many cases, spray pipes of the containment spray system are installed on two stages, i.e. upper stage and lower stage. In such cases, the lower-stage pipe is referred to as a lower dry well spray pipe, and the upper-stage pipe is referred to as an upper dry well spray pipe.
If an occurred event is not a LOCA but a transient such as a station blackout, water can be injected into the reactor pressure vessel 2 by the diverse high pressure core spray system 47 using the suppression pool water. However, the diverse high pressure core spray system 47 does not have a function of releasing the decay heat generated in the core 1 to the outside of the containment vessel 3.
The steam generated in the reactor pressure vessel 2 by the decay heat of the core fuel passes through a safety relief valve 72 and a discharge pipe 73 and then condensed in the suppression pool 6. Therefore, the decay heat of the core fuel is gradually transferred to the pool water of the suppression pool 6. As a result, the pool water of the suppression pool 6 rises in temperature.
As the pool water of the suppression pool 6 rises in temperature, saturated steam is generated in the wet well gas phase 7, driving the pressure of the wet well gas phase 7 higher than the pressure inside the dry well 4.
Accordingly, the gas (mainly including steam and nitrogen) inside the wet well gas phase 7 passes through the vacuum breaker 9, and gets into the dry well 4. As a result, the pressure inside the dry well 4 also rises, and the gas (mainly including nitrogen and steam) inside the dry well 4 is led to the heat exchanger 16 from the gas supply pipe 20. As a result, the non-condensable gas such as nitrogen holds up in the heat exchanger tubes 19 and the outlet plenum 18.
However, in the case of a transient in which the pool water of the suppression pool 6 rises in temperature, the pressure of the wet well 5 is higher than the pressure of the dry well 4. Therefore, the non-condensable gases that remain in the heat exchanger 16 cannot be vented into the wet well 5 by a pressure difference between the dry well 4 and the wet well 5.
Accordingly, when a transient such as station blackout should occur, the conventional passive containment cooling system 12 cannot cool the containment vessel 3. If the situation continues for a long time, the containment vessel 3 may fail by overpressure.
If the flash boiling of the pool water of the suppression pool 6 occurs by a rapid depressurization at containment failure, a pump of the diverse high pressure core spray system 47 is stopped. The pump of the diverse high pressure core spray system 47 may be stopped even when the suppression pool 6 itself fails by overpressure and water flows out.
As a result, the core may be damaged. At the very time of a station blackout the passive containment cooling system 12 is expected to perform its safety function because it does not require any external power source. However, there is a possibility that it cannot fulfill the safety function.
There is a plant, such as ESBWR, in which a depressurization valve (DPV) is placed on the reactor pressure vessel 2 to forcibly generate a gas phase break LOCA even at an occurrence of a transient. However, this is not a desirable option because the device extends a transient into a LOCA.
Therefore, an important challenge is to provide a passive containment cooling system 12 that can safely cool the containment vessel 3 without relying on such a depressurization valve even at an occurrence of a transient such as a station blackout.
Moreover, if the filtered venting system 50 is activated, the gas inside the containment vessel 3 is released into the filtered venting vessel 51. Therefore, the gas supply pipe 20 of the passive containment cooling system 12 cannot supply the gas inside the containment vessel 3 to the heat exchanger 16.
Accordingly, it becomes impossible to return the condensate into the containment vessel 3, and it is necessary to continue to inject water into the containment vessel 3 from an external water source. For example, if a residual heat removal system is so damaged by giant tsunami that the residual heat removal system cannot recover, it is necessary to continue to inject water from an external water source over a long period of time.
Therefore, an important challenge is to enable the passive containment cooling system 12 to cool the containment vessel 3 even when the filtered venting system 50 is activated, so that water does not need to be injected from an external water source.